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President Obama on Oct. 17 had his single most lucrative day of campaign fundraising ever, an Obama campaign official confirmed to ABC News.

The record-setting haul, which the official declined to specify, was attributed to a surge of online contributions from grassroots donors following Obama's performance in the second 2012 presidential debate.

The campaign announced last week that it has amassed a base of 4 million unique donors - or 1 in 75 Americans - a new record.

The flood of cash appears to signal a resurgent enthusiasm among Democrats, two weeks after the president's lackluster first debate rattled nerves and dampened spirits.

It's also a sign that the Obama's aggressive online fundraising operation continues to bear fruit. The campaign sent out at least six email appeals in a 12-hour period on the day after the second debate, including two messages from the president himself.

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"We send a lot of emails," wrote deputy Obama campaign manager Stephanie Cutter in a blast to supporters Thursday night. "I know it's a lot - but let me describe for you what the alternative looks like."

"Mitt Romney not only accepts big checks from these lobbyists, but as president he'd hire those same lobbyists in top-level positions," she said. "We could get the head of the junk food industry as the Secretary of Health and Human Services. An oil executive as the head of the Environmental Protection Agency."

Campaign officials have tried to combat complacency among donors all year by trying to instill a sense of fear about a President Mitt Romney. There are signs it has been an effective approach.

Obama and Democrats reported earlier this month they raised a whopping $181 million in September - more money than any incumbent president has ever raised in a single month for his re-election.

Romney and Republicans reported collecting more than $170 million in September. They have not disclosed the total number of contributors to their campaign.

The Republican presidential nominee enjoyed a burst of campaign cash following his strong first debate performance on Oct. 3. In the 48 hours after the debate, Romney collected north of $12 million, his campaign said.

Both campaigns will formally file their September fundraising reports with the Federal Election Commission on Saturday.

News of Obama's single biggest fundraising day was first reported by Politico's Mike Allen.